To define the nature of the disturbance created in peripheral nerve, the excitability of cutaneous and motor axons was monitored in 6 normal subjects requested to hyperventilate until paraesthesiae developed in the hands, face and trunk. This occurred when alveolar PCO2 (PACO2) had declined on average by 20 mmHg.

As PACO2 declined, the size of the compound sensory and muscle potentials evoked by a constant stimulus progressively increased, indicating an increase in axonal excitability. These changes occurred before paraesthesiae or tetany developed. In each subject there was a statistically significant inverse correlation between PACO2 and axonal excitability.

It is concluded that the paraesthesiae and tetany induced by hyperventilation result solely from changes in the excitability of cutaneous and motor axons in the peripheral nerve, presumably due to an alteration in the electrical properties of the axonal membrane resulting from a reduced plasma [Ca2+].

Abstract

The structural organization of peripheral nerves enables them to function while tolerating and adapting to stresses placed upon them by postures and movements of the trunk, head, and limbs. They are exposed to combinations of tensile, shear, and compressive stresses that result in nerve excursion, strain, and transverse contraction. The purpose of this appraisal is to review the structural and biomechanical modifications seen in peripheral nerves exposed to various levels of physical stress. We have followed the primary tenet of the Physical Stress Theory presented by Mueller and Maluf (2002), specifically, that the level of physical stress placed upon biological tissue determines the adaptive response of the tissue. A thorough understanding of the biomechanical properties of normal and injured nerves and the stresses placed upon them in daily activities will help guide physical therapists in making diagnoses and decisions regarding interventions.

Figure 1.

Structural components of peripheral nerves. In the endoneurial compartment (En), a single Schwann cell envelops several unmyelinated axons, and another Schwann cell provides multiple wrappings of plasma membrane forming the myelin sheath of a myelinated axon. The portion of a myelinated axon myelinated by a single Schwann cell is called the internode, and internodes are separated by nodes of Ranvier. Schwann cells associated with both unmyelinated and myelinated axons are covered with a continuous basal lamina (BL). Capillaries (Cap) are present within the endoneurial compartment, and collagen fibers (Col) run primarily longitudinally between the axons. The axons, Schwann cells, collagen, and endoneurial fluid are bundled into a fascicle by the perineurium (Pe). The perineurium consists of several layers of flattened perineurial cells connected by tight junctions and covered internally and externally by a basal lamina. The layers of perineurial cells are separated by collagen fibers (Col) oriented obliquely. Several fascicles are bundled together by the epineurium (Ep) to form a nerve. The epineurium consists primarily of fibroblasts, collagen fibers (Col), and elastic fibers. The epineurium between fascicles is termed the interfascicular epineurium, and that encompassing all of the fascicles is termed the epifascicular epineurium. Arterioles (A) and veins are oriented primarily longitudinally within the epineurium.

Blood supply

The blood supply to nerves is provided by coiled segmental arteries that enter the epineurium periodically along the length of the nerve and form the vasa nervorum. Arteries divide into epineurial arterioles that form an anastomotic network running primarily longitudinally within the epifascicular epineurium and the interfascicular epineurium (Fig. 3). Epineurial arterioles are supplied with a perivascular plexus of serotoninergic, adrenergic, and peptidergic nerves.17,18 Perforating arterioles cross the perineurium at oblique angles and carry a short sleeve of perineurial cells into the fascicle.3,19Perineurial arterioles have poorly developed smooth muscle and thus have limited ability to regulate intrafascicular blood flow.20Within the endoneurium, arterioles immediately turn into large-diameter, longitudinally oriented capillaries that allow blood flow in either direction (Fig. 3).21 The endothelial cells of endoneurial capillaries are connected by tight junctions, thus forming the tight blood-nerve barrier.7 Venules return blood to the venous system. Of note, lymphatic capillaries are present only within the epineurium; there is no lymphatic drainage from the intrafascicular or endoneurial space.22

Biomechanical properties

Under normal physiological conditions imposed by posture and movement, nerves are exposed to various mechanical stresses. Stress is defined as force divided by the area over which it acts9,23–25 and can be applied to a nerve as tensile, compressive, or shear stress or as a combination of stresses (Fig. 4). Tensile stress may be applied to tissues either parallel or perpendicular to the length of the nerve, causing respective longitudinal or transverse stress in the nerve. When joint motion causes elongation of the nerve bed, the nerve is inherently placed under tensile stress and accommodates the stress by both elongating and gliding.15 The deformation or change in nerve length induced by longitudinal tensile stress is called strain and is expressed typically as percent elongation.23,26–28 Displacement or gliding of a nerve relative to the surrounding nerve bed is called excursion.29–31 The direction of excursion may be longitudinal or transverse, or both, relative to the nerve tract31,32 and is measured in millimeters.

Figure 4.

Physical stresses placed on peripheral nerve. Tensile stress applied longitudinally to peripheral nerve creates an elongation of the nerve (an increase in strain). The transverse contraction that occurs during this elongation is greatest at the middle of the section undergoing tensile stress.

When the nerve bed is elongated, the nerve is placed under increased tensile stress. With the elongation of the nerve bed, the nerve glides toward the moving joint,1,33,34 a movement termed convergence.1 Conversely, if the nerve bed tension is relieved during joint motion, the nerve will realign along the shortened nerve bed by gliding away from the moving joint, a movement termed divergence.33 Convergence in the median nerve may be demonstrated during elbow extension (Fig. 5). The motion elongates the bed of the median nerve, causing the nerve segment in the arm to glide distally toward the elbow and the nerve segment in the forearm to glide proxi mally toward the elbow. In contrast, elbow extension relieves the tensile stresses in the ulnar nerve bed, causing the ulnar nerve to diverge away from the elbow (Fig. 5).

Figure 5.

Excursion of the median nerve (solid line) and the ulnar nerve (dotted line) during elbow extension followed by wrist extension. The concepts of nerve convergence toward and divergence away from a moving joint are illustrated in measurements of excursion taken at each site indicated. All measurements are reported in millimeters of proximal (P) or distal (D) excursion. The direction of excursion is also represented by solid arrows for median nerve excursion and open arrows for ulnar nerve excursion. (A) With elbow extension from 90° of flexion to 0° of flexion, the median nerve bed lengthens and the median nerve glides toward the elbow (converges). With the same joint motion, the ulnar nerve bed shortens and the ulnar nerve glides away from the elbow (diverges). (B) With wrist extension from 0° of extension to 60° of extension, both nerve beds lengthen; thus, both nerves converge toward the wrist. The magnitude of excursion is greatest closest to the moving joint. Data were obtained from: aDilley et al,29bWright et al,27 and cWright et al.33 Measurements of nerve excursion at the wrist and elbow in panel A were extrapolated from studies of nerve excursion during elbow flexion from 0° to 90°.27

Nerve Stiffness

First, a recent study43 measured greater nerve compliance in nerve segments that cross joints than in segments that do not cross joints.

Second, nerve stiffness is greater in long nerve sections and in nerve sections with numerous branches.15 Severing nerve branches or vessels but leaving the nerve in situ results in increased compliance and decreased stiffness.15

Third, nerve stiffness is greater when a nerve is elongated rapidly rather than slowly. In addition, the ultimate strain at the point of failure appears to be dependent on the rate of elongation.

When a nerve is placed under tension and maintained at that new fixed length over time, there is a reduction in the tension in the nerve or the force required to maintain the fixed length. The observed reduction in tension may be plotted in a stress-relaxation curve (Fig.8).25,44 The majority of relaxation occurs in the first 20 minutes of fixed elongation.25,44Stress relaxation in nerves that are stretched slowly is greater than in nerves that are stretched rapidly.25,37,44–46 This phenomenon was observed when comparisons were made for rabbit tibial nerves stretched at different rates to lengths 6% longer than their resting lengths. Over the 60-minute relaxation time, there was a 57% reduction in stress in nerves elongated at 0.08% per second,45 but only a 34% reduction in stress in nerves elongated at 3.0% per second.44

Figure 8.

Stress-relaxation curve demonstrating viscoelastic properties of peripheral nerve. When a nerve is elongated and the new length is kept constant, there is a rapid reduction in the stress within the nerve, expressed as percent reduced relaxation. Most of the relaxation occurs in the first 20 minutes. The degree of elongation affects the amount of stress relaxation that will occur. The dotted line represents a nerve that has been elongated to 6% above its resting length. The solid line represents nerves that have been elongated to 9% and 12% above their resting lengths. Greater stress relaxation was documented in nerves that underwent less elongation.25,44 Modified from Kwan MK, Wall EJ, Massie J, Garfin SR. Strain, stress, and stretch of peripheral nerve: rabbit experiments in vitro and in vivo. Acta Orthop Scand. 1992;63:267–272, with permission of Taylor and Francis AS.

However, a nerve stretched repetitively to 8% or 10% strain exhibits a reduced slope of the stress-strain curve, indicating that that nerve undergoes less stress with successive elongations because of increased compliance and decreased stiffness.

Compression of nerve

In addition to tensile stress, nerves are exposed statically and dynamically to compressive stresses. As mentioned previously, the laws of physics dictate that the cross-sectional area of a cylindrical object is reduced as the cylinder is elongated. As a nerve is elongated under tensile force, the nerve undergoes transverse contraction, which is resisted by the fluid and nerve tissue contained within the connective tissue sheath.15,39The magnitude of the transverse contraction stress is greatest at the center of the elongating segment15 (Fig. 4). Nerves also may be compressed externally by approximation to adjacent tissues, such as muscle, tendon, or bone, or by pressure increases in the extraneural environment. Compression of a nerve segment causes displacement of its internal contents in transverse and longitudinal directions. As shown in rat nerve, extraneural compression causes an immediate displacement of endoneurial fluid to the edges of a compressive cuff over 5 to 10 minutes and a much slower displacement of axonal cytoplasm over the course of hours.48 The damage to axons and myelin is greatest at the edges of the compressed zone,48,49 where the shear forces are highest.50

At the edges of the cuff, however, myelin retraction with resultant widening of nodes and paranodal demyelination occurred. These structural alterations in myelin may be expected to result minimally in impaired impulse conduction or maximally in demyelination and a conduction block.

In response to biomechanical stresses placed on a nerve as an individual assumes a posture or movement, the nerve follows the path of least resistance.29 Combinations of tensile, shear, and compressive stresses result in combinations of nerve excursion, strain, and transverse contraction. Because the biomechanical forces on the nerve are so intricately linked, the sequencing and range of joint movement affect the magnitude and direction of excursion,27,29 the magnitude of nerve strain,27,29,35 and the degree of transverse contraction at different sites along the nerve.27

Simultaneous nerve excursion, strain, and transverse contraction may be seen in the ulnar nerve as an example of responses to physical stresses imposed during movements of the upper limb. When the upper limb is maintained in a position of 90 degrees of shoulder abduction and 90 degrees of shoulder external rotation with the wrist neutral, and when the elbow is moved from 90 degrees of flexion to full extension, the ulnar nerve bed is shortened and the tensile stress on the nerve is decreased. With this motion, there is divergence of the ulnar nerve away from the elbow (Fig. 5), decreased nerve strain, especially at the elbow (Fig. 6), and decreased compression within the cubital tunnel.27,29,33 When the wrist then is extended from neutral to full extension, the ulnar nerve bed is lengthened, resulting in convergence of the nerve toward the wrist (Fig. 5), an increase in nerve strain (Fig. 6), and transverse contraction greatest in the nerve segment across the carpal bones and at the tunnel of Guyon.27,29,33 The magnitude of nerve strain and excursion will be greatest near the wrist, and the fascicles will rearrange as the nerve assumes a flattened oval shape. Because the nerve does not lie directly on the rotational axis of joint motion, the fascicles farthest from the axis will undergo greater strain than those closer to the center of rotation51

Figure 6.

Strain of the median nerve (solid line) and the ulnar nerve (dotted line) during elbow extension followed by wrist extension. Measurements at the sites indicated are reported as percent increase (↑) or percent decrease (↓) in strain. (A) With elbow extension from 90° of flexion to 0° of flexion, median nerve strain increases because of elongation of the nerve bed. Conversely, ulnar nerve strain decreases as the ulnar nerve bed shortens. (B) With wrist extension from 0° of extension to 60° of extension, the strain at the sites measured increases in both nerves as both nerve beds elongate. The magnitude of the strain is greatest closest to the moving joint. Data were obtained from: aWright et al27and bWright et al.33 Measurements of nerve excursion at the wrist and elbow in panel A were extrapolated from studies of nerve excursion during elbow flexion from 0° to 90°.27

Continuum of physical stress states

First, levels of physical stress lower than the levels required for tissue maintenance (low stress) result in a reduced ability of the tissue to tolerate subsequent stress and are consistent with tissue plasticity and response to functional demand.

Second, levels of physical stress in the range required for tissue maintenance (normal stress) result in no tissue adaptations and are considered to maintain a state of equilibrium.

Third, physical stress levels that exceed the range required for tissue maintenance (high stress) result in an increase in the tolerance of the tissue for stress in an effort to meet the mechanical demand.

Fourth, physical stress levels that exceed the capacity of some components of the tissue (excessive stress) result in tissue injury.

Fifth, levels of physical stress that are extreme (extreme stress) result in tissue death.

Finally, it is important to note that the physical stress level is a composite value with variable components of magnitude, time, and direction or type of stress.

In the functional zone, the physical stresses on the nerve are sufficient to maintain a state of equilibrium and normal physiological function. In the dysfunctional zone, various levels of physical stress have altered the ability of the nerve to tolerate subsequent stress.

Figure 9.

Continuum of physical stress states. The white area represents the functional zone in which the physical stresses on the nerve are sufficient to maintain a state of equilibrium and normal physiological function. The shaded areas represent dysfunctional zones resulting from various levels of physical stress placed on the nerve tissue. Under conditions of prolonged low stress, the functional zone will shrink in width and shift to the left, reducing the ability of the tissue to tolerate subsequent stresses even of previously normal levels. Under conditions of high stress, the functional zone may expand and shift to the right, improving the ability of the tissue to tolerate subsequent physical stress. If the nerve is exposed to prolonged or repeated excessive stress, the functional zone will shrink in width. Although scarring of damaged tissue may enable the nerve to tolerate subsequent physical stresses, the physiological function of the nerve will be reduced. Exposure to extreme stress will result in disruption of axon continuity or neural cell death and significantly reduced physiological function.

Immobilization Stresses

Under conditions of immobilization, such as casting, splinting, and bracing, peripheral nerves are exposed to levels of physical stress that are lower than those necessary to maintain the nerves in a state of equilibrium or in a functional zone (Fig. 9). According to the Physical Stress Theory, nerve will undergo predictable physiological and structural modifications proportional to the levels of reduced stress and the duration of immobilization.2 Immobilization induces cell biological changes in axons and axon terminals52–54 and structural changes in myelin and nerve connective tissue layers that likely alter the ability of nerves to tolerate subsequent physical stress.

We hypothesize that after a period of immobilization, the width of the functional zone on the continuum of physical stress states will shrink and shift toward the left (Fig. 9).

Lengthening Stresses

Nerves are exposed to various levels of longitudinal tensile stress during limb-lengthening procedures, such as distraction osteogenesis (Ilizarov procedures), traction injuries, and stretching maneuvers. The tissue response is dependent upon the magnitude and duration of the tensile stress. The extant data indicate that lengthening of 6% to 8% for a short duration causes transient physiological changes that appear to be within the normal stress tolerance of the tissue, whereas acute strains of 11% and greater cause long-term damage and may be considered to be excessive or extreme stress states.

In cadavers, positioning in shoulder depression, 90 degrees of shoulder abduction, 90 degrees of shoulder external rotation, 70 degrees of forearm supination, 60 degrees of wrist extension, full finger extension, and full elbow extension resulted in 7.6%±8.2% (X̄±SD) strain in the median nerve measured just proximal to the wrist.28 Adults who were healthy and who were placed in this same position lacked 12±13 degrees (X̄±SD) of elbow extension because of substantial discomfort in the limb.61The subjects reported pain of 5.1±1.9 (X̄±SD) on a 10-point visual analog scale, and 36% of the subjects reported paresthesia in the upper limb. Taken together, these findings suggest that many people are unable to tolerate levels of strain below the theoretical 11% threshold.

Compression Stresses

Compression on a nerve may be the result of extraneural force or may occur as transverse contraction secondary to increased longitudinal strain (Fig. 4). Compression stress of a low magnitude and a short duration may result in reversible physiological and minor structural changes. Compressive stress of a high magnitude, however, may result in structural alterations in myelin sheaths and even disruption of axons. Low-magnitude compressive stress applied over a long period of time may cause significant structural changes in the nerve secondary to impairment of blood flow and sequelae of ischemia.

As with strain-induced injury, a threshold for compression-induced nerve injury is difficult to determine. Common functional positions may result in compression pressures that approach or exceed the 20 to 30 mm Hg demonstrated to impair nerve blood flow.75The carpal tunnel is a site well known for compressive damage to the median nerve and thus has been well studied. Carpal tunnel pressure in subjects who were healthy was measured at 3 to 5 mm Hg with the wrist in a neutral position.76–78Simply placing the hand on a computer mouse was shown to increase the tunnel pressure from the resting 5 mm Hg to 16 to 21 mm Hg,79and actively using the mouse to point and click increased the tunnel pressure to 28 to 33 mm Hg, a pressure high enough to reduce nerve blood flow.

In subjects with carpal tunnel syndrome, pressure in the carpal tunnel was 32 mm Hg with the wrist in a neutral position and rose to a mean of 110 mm Hg with full wrist extension in subjects with carpal tunnel syndrome.76 These tunnel pressures exceed the threshold of 20 to 30 mm Hg for vascular perfusion even at rest. Taken together, these findings suggest that even functional positions, such as the use of a computer keyboard and mouse, place the wrist in a position of increased carpal tunnel pressure, compromising nerve blood flow and placing people at risk for median nerve injury.

Direct damage to myelin and axons has been shown to occur with extraneural compression of as low as 50 mm Hg maintained for 2 minutes,48 and the percentage of damaged fibers increases with increasing force. Ten days after the application of compressive stress at 50 mm Hg, 30% of the axons in the region under the compressive cuff showed evidence of demyelination, focal myelin thickening, remyelination, and axonal degeneration or regeneration.48

The pathological consequences of prolonged compression include subperineurial edema; inflammation; deposition of fibrin; activation of endoneurial fibroblasts, mast cells, and macrophages; demyelination; axon degeneration; and fibrosis.83 Compression of a very long duration has been modeled in animals with loose ligatures,88 Silastic* tubes,89,90and pressure balloons placed within an anatomical tunnel.91 The pathological findings are thought to result from both inflammatory and cellular phenomena and include changes in the blood-nerve barrier, thickening of the perineurium and epineurium, thinning of myelin, demyelination and degeneration of axons in the fascicle periphery, and slowed nerve conduction velocity.

In the case of chronic compression, decompression is paramount. Physical therapy intervention should focus on reduction of inflammation, improvement in blood flow, and enhancement of the capacity of the nerve for strain and excursion along its full length in an effort to reduce the physical stress on the compressed region.

Repetitive Stresses

Vibration constitutes one form of repetitive stress. We know from studies of humans who use hand-held vibrating tools that vibration stresses can cause reductions in tactile sensation, as well as other sensory disturbances96 and reduced grip force.97,98Furthermore, myelin breakdown and fibrosis have been seen in the dorsal interosseous nerve at the wrist in people with vibration-induced neuropathy.99 Long-term exposure to vibration stresses has been shown to result in the grouping of muscle fiber types in muscle biopsies, indicative of denervation and reinnervation.98

Repetitive movements, such as those that occur in work-related musculoskeletal disorders, were discussed in detail recently by Barr and Barbe.102 The stresses placed upon the tissues may be variable in terms of type, magnitude, frequency, and duration, and the combination of these factors may place nerves in normal to extreme levels of physical stress. The chronic inflammation associated with repetitive movements places nerves under constantly higher hydrostatic compressive stress, which may increase further with contraction of the surrounding muscles. Chronic inflammation elicits within the nerves a remodeling response that seeks to add mechanical stability.103The most common outcome is the deposition of collagen in the connective tissue layers, which leads to decreased compliance of the nerves to elongation. As with chronic compression, the approach for assessment and treatment of injuries attributable to repetitive movements must address the chronic inflammatory state and connective tissue changes. Of primary importance in interventions for all stress-induced injuries are the identification and characterization of physical stresses and the modification of their components, magnitude, time, and direction, as outlined in the physical stress theory.2

Summary

This assessment should guide treatment interventions to normalize the stresses on the nerves, be they rest, soft tissue or neurodynamic mobilization, stretching, modalities, exercise, or patient education. Treatment rationale should be based on an educated understanding of the biomechanical properties of normal and pathological nerves. The concept of a continuum of low-normal-high-excessive-extreme stresses may be used as a training tool for patient education, pointing out examples of daily activities that fall under the different categories.

Abstract

OBJECTIVES:

Prior studies have found that a breathing pattern of 6 or 5.5 breaths per minute (bpm) was associated with greater heart rate variability (HRV) than that of spontaneous breathing rate. However, the effects of combining the breathing rate with the inhalation-to-exhalation ratio (I:E ratio) on HRV indices are inconsistent. This study aimed to examine the differences in HRV indices and subjective feelings of anxiety and relaxation among four different breathing patterns.

METHODS:

Forty-seven healthy college students were recruited for the study, and a Latin square experimental design with a counterbalance in random sequences was applied. Participants were instructed to breathe at two different breathing rates (6 and 5.5 breaths) and two different I:E ratios (5:5 and 4:6). The HRV indices as well as anxiety and relaxation levels were measured at baseline (spontaneous breathing) and for the four different breathing patterns.

RESULTS:

The results revealed that a pattern of 5.5 bpm with an I:E ratio of 5:5 produced a higher NN interval standard deviation and higher low frequency power than the other breathing patterns. Moreover, the four different breathing patterns were associated with significantly increased feeling of relaxation compared with baseline.

CONCLUSION:

The study confirmed that a breathing pattern of 5.5 bpm with an I:E ratio of 5:5 achieved greater HRV than the other breathing patterns. This finding can be applied to HRV biofeedback or breathing training in the future.

These diseases include cardiovascular disease, cancer, Alzheimer’s disease and the metabolic syndrome. Three basic mechanisms contribute to such illnesses: local oxidative stress and DNA damage, inflammatory reactions and excessive sympathetic responses, all of which are inhibited by vagal nerve activity.

Recent evidence reveals that the immune system is under the direct control of the vagus nerve via the “cholinergic anti-inflammatory pathway.” Stimulation of vagus nerve activity significantly inhibits cytokine levels in animal models, and cholinergic agents inhibit cytokine release by human macrophages. Moreover, when vagus nerve activity is decreased or absent, cytokines are overproduced. Atherosclerosis is an inflammatory disease characterized by elevated levels of CRP and IL-6, but the relationship between cardiac vagal activity and cytokine levels in healthy humans is not well understood. Here we measured RR interval variability, an index of cardiac vagal modulation, and CRP and IL-6 in 757 subjects participating in a subset of the year 15 data collection in the CARDIA study of the evolution of risk factors in young adults. Univariate analysis revealed that all indices of RRV were strongly and inversely related to IL-6 (log pg/mL b= −0.08 and −0.17 for HF and LF power, P < 0.001 respectively) and CRP (log mg/L b = −0.14 and −0.26 for HF and LF power, P < 0.001 respectively) levels. In the multivariate model including gender, race, age, smoking, physical activity, SBP, BMI, and disease, the inverse relationship between RRV and inflammatory markers, although slightly attenuated, remained significant. These findings are consistent with the hypothesis that diminished descending vagal anti-inflammatory signals can allow cytokine overproduction in humans.

To our knowledge, these are the first results demonstrating inverse relationships between inflammatory markers and indices of cardiac autonomic regulation in a large sample of healthy young adults. These findings are consistent with evidence from animal studies indicating that the cholinergic anti-inflammatory pathway counter-regulates inflammation.

It now appears that in our data from the CARDIA study of heart disease in young adults there is an inverse relationship between low frequency RR interval variability and the inflammatory markers IL-6 and CRP, even after control of relevant covariates and cardioactive medications or hypertension or diabetes, which is consistent with the hypothesis of a cholinergic anti-inflammatory pathway that regulates inflammation in humans.

Objectives. A central hypothesis of the cholinergic anti-inflammatory reflex model is that innate immune activity is inhibited by the efferent vagus. We evaluated whether changes in markers of tonic or reflex vagal heart rate modulation following behavioural intervention were associated inversely with changes in high-sensitivity C-reactive protein (hsCRP) or interleukin-6 (IL-6).

Conclusions. Changes in hsCRP were consistent with the inhibitory effect of increased vagal efferent activity on pro-inflammatory factors predicted by the cholinergic anti-inflammatory reflex model. Clinical trials for patients with cardiovascular dysfunction are warranted to assess whether behavioural interventions can contribute independently to the chronic regulation of inflammatory activity and to improved clinical outcomes.

Chronic low-grade inflammation contributes to the development of experimental and clinical hypertension [1–3], and it increases the risk for myocardial infarction, stroke and sudden cardiac death [4]. C-reactive protein (CRP) is an established index of systemic inflammation. It is produced chiefly by hepatocytes under the regulation of a cascade of pro-inflammatory cytokines [tumour necrosis factor-α (TNF-α), interleukin-1ß [IL-1ß] and IL-6] that are expressed in response to conditions that include vascular injury and infection. In addition, CRP is produced by human coronary artery smooth muscle cells following exposure to pro-inflammatory cytokines [5], which suggests that it may contribute independently to endothelial dysfunction and atherogenesis [6].

Clinical trials that have attempted to modify vagal efferent activity by means of aerobic exercise [17, 18], resistance exercise [19] or device-guided vagal nerve stimulation [20–22] have yet to demonstrate consequent reduction in pro-inflammatory activity that is independent of confounding factors such as anti-inflammatory medications.

Subjects received four weekly and two biweekly 1-h sessions of behavioural neurocardiac training or autogenic relaxation, as described previously [23]. Home practice sessions complemented the laboratory-based training. All sessions began with a 10-min review of cognitive-behavioural guidelines for managing daily stress [25].

At the completion of each task, subjects were trained to cognitively disengage from negative or aroused affect and to focus attention on slowing respiration (within their comfort zone) to 10-s cycles (6 breaths min−1). During each countering exercise, subjects were guided by the use of biofeedback to increase RR spectral power at approximately 0.1 Hz, as shown on a biofeedback display of the RR power spectrum (0.003–0.5 Hz) and breaths min−1.

The major finding of this study is that following an 8-week protocol of behavioural neurocardiac training or autogenic relaxation amongst patients with hypertension, change in hsCRP was associated independently and inversely with changes in tonic and reflex vagal heart rate modulation as measured by RR high-frequency power (ms2 per Hz), baroreflex sensitivity (ms per mmHg) and lengthening of the RR interval (ms). A statistical trend in the data suggested a similar inverse association between changes in IL-6 and RR high-frequency power.

A central hypothesis of the cholinergic anti-inflammatory reflex model is that the innate immune response is regulated, in part, by rapid and localized efferent activity of the vagus nerve. Previous reviews have identified the functional anatomy and neural mechanisms of this model [10, 29, 30]. In brief, efferent fibres of the vagus nerve comprise a neural anti-inflammatory pathway that culminates in the release of acetylcholine in proximate sites where pro-inflammatory factors have been expressed. Acetylcholine has been shown to bind to subunit α7 of nicotinic acetylcholine receptors on cytokine-producing immune cells [30]. This inhibits the activation of NF-κB and the subsequent expression of a pro-inflammatory cascade that includes TNF-α, IL-6 and CRP [10].

To our knowledge, the present proof of principle study involving hypertensive patients provides the most direct evaluation of whether augmentation of tonic or reflex vagal heart rate modulation, in this instance by a behavioural intervention, attenuates independently pro-inflammatory activity as assessed by hsCRP and IL-6. It is noteworthy that the present findings were observed following only modest changes in markers of vagal HR modulation. Previous behavioural trials of heart rate variability biofeedback or relaxation [32–34] have reported a small but statistically significant increase in vagal HR modulation. Similarly, behavioural training is associated with a modest, but statistically significant decrease in proinflammatory factors, including hsCRP and IL-6 [35], although heart rate variability biofeedback failed to reduce other inflammatory factors following experimental administration of an endotoxin (lipopolysaccharide) [36].

In sum, the present findings support the model of a cholinergic anti-inflammatory reflex when pro-inflammatory activity is measured by hsCRP. Clinical trial evidence has demonstrated that behavioural interventions can significantly augment vagal heart rate modulation or cardiovagal baroreflex gain through the use of relaxation training and biofeedback [32–34].